The majority of international transport of goods takes place by means of containers. Containers are standard-shaped transport units in which goods are packed for the duration of transport. Typically, containers come in three different sizes of 20 feet, 40 feet or 45 feet in length. A container is about 2.5 meters wide.
Containers are handled at a container terminal (either in ports or inland) by particular container cranes, which include rail mounted gantry cranes (RMG cranes) and rubber-tyred gantry cranes (RTG cranes). A particular type of rail mounted gantry crane in a container port is a ship-to-shore crane used for lifting containers to be unloaded from a ship onto a quay and, correspondingly, for loading containers brought to a quay on board a container ship.
For handling containers a container crane typically has a specific container grabber (spreader) mounted thereto for gripping and lifting a container. The length of a spreader may be altered according to the length of the container being handled.
When containers are handled, they are typically loaded one on top of the other or carried from one stack to another. Stacking of containers requires great precision. The stacks may have a plurality of containers, e.g. five, and therefore imprecisions in the stacking of the containers may cause the entire stack to collapse and thus endanger the staff and equipment around the stack.
When containers are handled, the grabber may be subjected to vibration and collisions from various sources. Vibration and collisions typically occur for example when the grabber is placed on top of a container and it grabs the container. Vibration and collisions complicate the positioning of containers with sufficient precision. On the other hand, attenuation of vibration may take a long time, which delays container handling and decreases the efficiency of the handling. Collisions may take place at an acceleration of the order of 1000 m/s2, i.e. about 100 G.
Crane operation is increasingly automated to speed up container handling. In automated container handling the driver may monitor the handling on a display in the crane cabin or over a remote connection from an office, for example. The driver does not need to monitor the handling all the time, but the automation system may alert the driver when his/her attention is required. Since the driver does not have a direct visual contact with the container to be handled, precise operation and reliability of the equipment to be used for automated handling are an essential aspect with regard to the efficiency of container handling. Servicing or malfunction of the automation equipment causes service breaks in the crane operation and prevents handling of containers. From the point of view of the crane owner, service breaks are expensive and cause congestion in container handling at a crane terminal, for example.